web: don't bother building hledger-web with GHC 7 on a mac

GHC 8.0+ is needed for mac Sierra+.

hledger-web/package.yaml

@@ -127,11 +127,18 @@ library:
   - wai-handler-launch >=1.3
   - warp
   - yaml
-  - yesod        >=1.4 && < 1.7
-  - yesod-core   >=1.4 && < 1.7
-  - yesod-form   >=1.4 && < 1.7
-  - yesod-static >=1.4 && < 1.7
   - HUnit
+  # on mac Sierra or greater, ghc 7 will fail to build yesod; don't try  
+  when:
+    - condition: os(darwin) && impl(ghc < 8.0)
+      then:
+        buildable: false
+      else:
+        dependencies:
+        - yesod        >=1.4 && < 1.7
+        - yesod-core   >=1.4 && < 1.7
+        - yesod-form   >=1.4 && < 1.7
+        - yesod-static >=1.4 && < 1.7
 
 executables:
   hledger-web:
@@ -146,6 +153,9 @@ executables:
       buildable: false
     - condition: flag(threaded)
       ghc-options: -threaded
+    when:
+    - condition: os(darwin) && impl(ghc < 8.0)
+      buildable: false
 
 # 2018/06 not building and not testing anything useful, disabled for now
 # tests: